1. Field of the Invention
The present invention relates to a triple-effect absorption refrigeration system. Particularly, the present invention relates to a triple-effect absorption refrigeration system using a direct-fired heat source to heat its high temperature generator. More particularly, the present invention relates to cooling the combustion chamber of the direct-fired heat source with a sub-ambient pressure solution stream.
2. Description of the Related Art
Absorption refrigeration systems are generally used to cool commercial buildings. A single-effect absorption refrigeration system typically comprises a generator, a condenser, an evaporator and an absorber. In this system, a refrigerant-containing absorption solution stream is heated in the generator by an outside heat source, such as a fuel burner, low-pressure steam, or hot water, in order to boil off refrigerant vapor. The refrigerant vapor is condensed to refrigerant liquid, and then routed to an evaporator. The refrigerant liquid in the evaporator absorbs the heat from the air in the commercial building being cooled, again flashing to vapor. The refrigerant vapor flows to an absorber, where it mixes with an absorption liquid, and the combined refrigerant-containing absorption solution stream is pumped to the generator.
The single-effect absorption system described above is extremely inefficient, having a thermal Coefficient of Performance (COP) of approximately 0.7.
A more efficient approach is to use a double-effect absorption refrigeration system. In this latter system, the single generator and condenser are replaced by two generators (a high temperature generator and a low temperature generator) and two condensers (also high temperature and low temperature). Primary heat is supplied to the high temperature generator to boil off refrigerant vapor from the refrigerant-containing absorption solution stream. The refrigerant vapor is condensed in the high temperature condenser. The heat of condensation from the high temperature condenser is used to heat the refrigerant-containing absorption solution stream in the low temperature generator, boiling off more vapor in that generator. In this manner, the heat input to the system is utilized twice to generate refrigerant vapor. The thermal COP of a double-effect absorption system hence is improved to approximately 1.2.
First stage generators of double-effect chillers may be either steam-driven or direct-fired. In commercially produced direct-fired double-effect chillers, a burner is used to produce high temperature combustion gases which heat the solution stream in the high temperature generator. Typically, the combustion chamber of a direct-fired double-effect chiller is a Scotch Marine-type boiler, where the combustion chamber and the generator are built as one component. Combustion takes place inside the combustion chamber, while combustion air flows through the chamber. Heat from the combustion air is transferred to the solution stream in the generator, thereby cooling the combustion air and boiling the solution stream. One advantage of this arrangement is that the combustion chamber wall is cooled by the solution stream on the other side, decreasing the temperature of the flame and the combustion wall. Heat exchanger tubes may be installed inside the combustion chamber to increase the heat transfer rate between the combustion air and the solution stream.
Conventional double-effect absorption chillers operate at sub-ambient pressure. Thus, although with a Scotch Marine-type boiler, the flame directly contacts the common combustion chamber/generator wall, the chamber is exempt from the ASME Boiler and Pressure Vessel Code, Section 1, entitled "Power Boilers" (ASME, Section 1), because the boiling of the solution stream occurs at a pressure less than ambient.
In recent years, experiments have been conducted with triple-effect absorption systems, utilizing three generators and three condensers with a single absorber and a single evaporator. In a triple-effect absorption system, the single generator and condenser are replaced by three generators (a high temperature generator, an intermediate temperature generator, and a low temperature generator) and three condensers (also high temperature, intermediate temperature, and low temperature). First, primary heat is supplied to the high temperature generator to boil off refrigerant vapor from the refrigerant-containing absorption solution stream. The refrigerant vapor is condensed in the high temperature condenser. Second, the heat of condensation from the high temperature condenser is used to heat the refrigerant-containing absorption solution stream in the intermediate temperature generator, boiling off refrigerant vapor in that generator. The refrigerant vapor from this intermediate temperature generator is then condensed in the intermediate temperature condenser. Third, the heat of condensation from this intermediate temperature condenser is used to heat the refrigerant-containing absorption solution stream in the low temperature generator, boiling off refrigerant vapor in that generator. Thus, triple-effect absorption systems use the primary heat input to the high temperature generator three times to generate refrigerant vapor.
FIG. 1 depicts a triple-effect absorption system which is fully described in U.S. patent application Ser. No. 08/743,373, filed Nov. 4, 1996, titled "Triple Effect Absorption Refrigeration System," the disclosure of which is incorporated herein by reference. An absorber A provides refrigerant-containing absorption solution stream to three generators, including a high-temperature generator HTG, an intermediate-temperature generator ITG, and a low-temperature generator LTG. Each generator feeds refrigerant vapor to a corresponding condenser, including a high-temperature condenser HTC, an intermediate-temperature condenser ITC, and a low-temperature condenser LTC. Furthermore, the high-temperature condenser HTC is coupled with the intermediate-temperature generator ITG, and the intermediate-temperature condenser ITC is coupled with the low-temperature generator LTG. Hence, the system is referred to as a double-coupled condenser (DCC) triple-effect absorption system. Heat exchangers HX1, HX2, and HX3 may be provided in the flowpath of the solution stream from the absorber.
The primary energy of the triple-effect absorption system is input to high-temperature generator HTG, where it heats the absorption solution stream and generates refrigerant vapor as described below. The refrigerant vapor generated from high-temperature generator HTG is condensed in high-temperature condenser HTC, and the heat of condensation is exchanged with intermediate-temperature generator ITG in order to generate refrigerant vapor in intermediate-temperature generator ITG, as described above. The condensate from high-temperature condenser HTC and the vapor from intermediate-temperature generator ITG pass into intermediate-temperature condenser ITC. The heat of condensation from intermediate-temperature condenser ITC is exchanged with low-temperature generator LTG in order to generate refrigerant vapor from low-temperature generator LTG as described above. The condensate from intermediate-temperature condenser ITC and the vapor from low-temperature generator LTG collect in low-temperature condenser LTC, and the resulting condensate is sent to evaporator E in order to obtain the desired refrigeration effect. The resultant low pressure vapor is then passed from evaporator E to absorber A, where it combines with the returning concentrated absorption solution stream to dilute the solution stream and begin a new cycle.
The triple-effect absorption system of FIG. 1 has an inverse parallel series solution feeding arrangement. Low-temperature generator LTG and intermediate-temperature generator ITG are connected to absorber A in a parallel flow arrangement, and high-temperature generator HTG is connected to intermediate-temperature generator ITG in a series flow arrangement. A weak absorption solution stream is fed from the absorber to the low-temperature generator LTG and to the intermediate-temperature generator ITG in parallel. The solution stream in intermediate-temperature generator ITG is heated, and refrigerant vapor is boiled off. The now more concentrated solution stream exits intermediate-temperature generator ITG and flows to high-temperature generator HTG. This solution stream is further concentrated as more refrigerant is boiled off, and then exits high-temperature generator HTG. Likewise, the weak solution in low-temperature generator LTG is concentrated as refrigerant is boiled off. This more concentrated solution stream exits low-temperature generator LTG, mixes with the solution stream exiting high-temperature generator HTG, and returns to absorber A. Computer simulations show that the inverse parallel series solution stream feeding arrangement of FIG. 1 provides a thermal COP of 1.720.
In order to achieve high performance levels, triple-effect absorption cycles operate at higher temperatures than double-effect or single-effect cycles. Typically, triple-effect cycles are calculated to operate with solution stream temperatures in the high temperature generator that are 100.degree. F. to 150.degree. F. higher than in a double-effect cycle. For example, typical double-effect refrigeration systems, using a lithium bromide-water solution stream, operate with solution stream temperatures leaving the high temperature generator of 300.degree. F. to 330.degree. F., while analysis of similar triple-effect refrigeration systems shows they would operate with solution stream temperatures leaving the high temperature generator of 400.degree. F. to 480.degree. F. As a result, triple-effect chillers will most likely be direct-fired.
The operating pressure of a triple-effect absorption system may be above or below ambient, depending on whether the system is absorber-coupled or condenser-coupled. If the system is absorber-coupled, as in a dual-loop cycle, then the operating pressure may be lower than ambient. One drawback to an absorber-coupled triple-effect system is that it requires an absorption solution having a much wider solubility than is available from the conventional absorption solution of lithium bromide-water. If the system is condenser-coupled, as in a pressure-staged triple-effect cycle, then the operating pressure is higher than ambient. The advantage of the condenser-coupled system is that it may use the conventional absorption solution of lithium bromide-water.
Most triple-effect systems considered for commercial applicability operate at a first stage generator pressure much higher than ambient pressure. The inverse parallel series flow cycle disclosed in U.S. patent application Ser. No. 08/743,373, noted above, is one example of a condenser-coupled, pressure-staged system. As predicted by thermodynamic analysis, the typical first stage pressure of this system is around 60 psia. However, even though the first stage generator of a condenser-coupled system typically operates above ambient pressure, the second stage generator typically operates under vacuum.
Because of their above ambient operating pressures, the first stage generators of condenser-coupled triple-effect chillers fall under the jurisdiction of ASME, Section 1, if they are directly contacted by the combustion chamber flame. One of the requirements of ASME, Section 1 is that such generators must be attended by a full time operator. The labor cost associated with such a full time operator is prohibitive and significantly affects the commercial viability of such condenser-coupled triple-effect chillers. Also, complying with ASME, Section 1 requires thicker construction, more safety devices, and the need to conform to Pressure Vessel codes. All of these requirements translate into higher design and fabrication costs.
One possible solution is to use an adiabatic combustion chamber in the direct-fired triple-effect chiller, thereby exempting its high pressure, first stage generator from ASME, Section 1. With an adiabatic combustion chamber, combustion and boiling take place in two different vessels. Combustion occurs in the adiabatic chamber which is insulated with refractory lining. No solution stream enters the chamber, thus, no boiling takes place in the combustion chamber. Rather, the combustion gases are directed to the first stage generator, where the high pressure solution stream absorbs heat from the gases and generates high pressure vapor. As long as the piping arrangement between the adiabatic combustion chamber and the high pressure generator ensures that the flame of the direct-fired burner does not contact the generator, the generator will not be governed by ASME, Section 1 requirements.
However, adiabatic combustion chambers have certain disadvantages. First, an adiabatic combustion chamber requires a fairly thick, high temperature refractory insulation, which is very expensive. Second, the high temperature radiant environment within an adiabatic chamber increases nitrous oxide (NOx) emissions. Increasing air flow through the chamber reduces the amount of NOx emissions, but increasing air flow also reduces the combustion efficiency. Thus, the cost and the NOx emission levels of such adiabatic combustion chambers are expected to be too high.